1. Field of the Invention
The present invention relates to a method for manufacturing semiconductor devices, in particular, a method for manufacturing active matrix display devices.
2. Description of the Related Art
Heretofore, liquid crystal display devices utilizing, as switching elements, TFTs each formed using amorphous silicon have been often used as display devices which have been widely used, such as liquid crystal televisions, displays of personal computers, and cellular phones. A technique by which a TFT is formed using a semiconductor thin film formed over a substrate having an insulating surface has attracted attention. The TFT has been widely applied to electronic devices such as ICs and electro-optical devices and particularly developed as a switching element of an image display device.
For a TFT using amorphous silicon, a layered structure has been conventionally formed through a photolithography process using five or more photomasks. Reduction in photolithography process using photomasks has been desired. Heretofore, each of Patent Document 1 (Japanese Published Patent Application No. 2000-131719) and Patent Document 2 (Japanese Published Patent Application No. 2003-45893) has been known as a technique achieving reduction in number of steps in a photolithography process using photomasks.
FIGS. 10A to 10E are structural views illustrating a conventional TFT using amorphous silicon.
The manufacturing process thereof is described. A gate electrode 501 is formed over a glass substrate 500 by a photolithography step using a first photomask (FIG. 10A).
A gate insulating film 502, an i-type amorphous silicon layer 503, and an n+-type amorphous silicon layer 504 are formed. The i-type amorphous silicon layer 503 and the n+-type amorphous silicon layer 504 form an island region by a photolithography step using a second photomask (FIG. 10B).
A source electrode 508 and a drain electrode 509 are formed by a photolithography step using a third photomask. At that time, a photoresist formed by the third photomask is successively utilized to etch the n+-type amorphous silicon layer so that a channel region 505, a source region 506, and a drain region 507 are formed.
A protective film 510 is formed, and a contact hole through which a contact with a pixel electrode 511 is made is formed by a photolithography step using a fourth photomask (FIG. 10C).
Indium tin oxide (ITO) is formed, and the pixel electrode 511 is formed by a photolithography step using a fifth photomask (FIGS. 10D and 10E).
Photolithography steps using a photomask includes application of a photoresist, pre-baking, a step of light exposure using a metal photomask, a step of development, post-baking, a step of etching, a step of resist separation, and the like. In addition, many steps such as a step of cleaning and a step of inspection are included in the photolithography steps. Thus, performing the conventional process using five photomasks means that the steps are repeated five times, which is a significant factor in the decrease in throughput in the manufacturing process or the increase in manufacturing cost.